(a) Field of the Invention
The invention relates to cDNA probes and antibodies for the detection of human methenyltetrahydrofolate synthetase (MTHFS) in biological tissue samples to determine the appropriate chemotherapy for a given patient treated with the reduced folate Leucovorin.TM..
(b) Description of Prior Art
Many plant and animal tissues contain folic acid (pteroylglutamic acid) as the polyglutamates of the reduced tetrahydro forms. These folates act as co-enzymes for processes in which there is transfer of a one-carbon unit (e.g., in purine and pyrimidine nucleotide biosynthesis), amino acid conversions (e.g., histidine to glutamic acid through forminoglutamic acid), and generation and use of formate.
Absorption of folic acid takes place in the small intestine. In the epithelial cells, folic acid are reduced to dihydro- and tetrahydrofolates. They are bound to protein and transported as methyl tetrahydrofolate. Serum levels vary from 3 to 21 ng/mL and closely reflect dietary intake. Red blood cell (RBC) folate, normal 160 to 640 ng/ml whole blood (corrected to packed cell volume of 45%), is a better indicator of the folate status. The total body folate is about 70 mg, 1/3 of which is found in the liver. About 20% of ingested folate is excreted unabsorbed together with 60 to 90 .mu.g/day of ingested folate not reabsorbed from bile. Reduced folates are present in all living tissues although decreased amounts of certain reduced folates in cancer cells are thought to be responsible for the inefficacy of the anti-cancer agent 5-fluorouracil (Ullman, B. et al., Proc. Natl. Acad. Sci. USA, 75:980-983, 1978). 5-formyltetrahydrofolate is thus administered clinically, as Leucovorin.TM., in association with 5-fluorouracil to enhance its cytotoxic effects through amplification of intracellular reduced folate pools (Grem JL et al., Cancer Treat. Rep., 71:1249-64, 1987).
Methenyltetrahydrofolate synthetase (MTHFS; 5-formyltetrahydrofolate cyclodehydrase, EC 6.3.3.2), a 27 KDa monomer, catalyzes the unidirectional transformation of 5-formyltetrahydrofolate to 5-10-methenyltetrahydrofolate and requires ATP and divalent cations. MTHFS activity has been purified from sheep liver, Lactobacillus casei, rabbit and human liver (Bertrand R. et al., Bioch. Biophys. Acta, 911:154-61, 1987).
MTHFS is the obligatory initial metabolic step prior to 5-formyltetrahydrofolate's intracellular conversion to other reduced folates and one experimental model suggests that variations in MTHFS activity between different tissues can influence intracellular 5-formyltetrahydrofolate interconversion to other reduced folates and its ability to enhance 5-fluorouracil activity (Mullin R. J. et al., "The expanding role of folates and fluoropyrimidines in Cancer Chemotherapy", Plenum, New York, pp. 25-38, 1988). The enzyme might thus play a role in the cellular pharmacology of Leucovorin.TM..
Physiologically, 5-formyltetrahydrofolate is produced through hydrolysis of 5-10-methenyltetrahydrofolate by a serine hydroxymethyltransferase-glycine complex (Stover P. et al., J. Biol. Chem., 265:14227-14233, 1990). The 5-formyltetrahydrofolate polyglutamates thus formed can then directly inhibit serine hydroxymethyltransferase, the main source of one carbon residues for folate-dependent syntheses (Stover P. et al., J. Biol. Chem., 266:1543-1550, 1991).
Inhibitory studies have demonstrated that MTHFS prevents 5-formyltetrahydrofolate polyglutamates from accumulating intracellularly and inhibiting de novo purine synthesis (Bertrand R. et al., J. Biol. Chem., 264:8843-6, 1989). MTHFS might thus also play an important physiological function in eliminating inhibitory 5-formyltetrahydrofolate polyglutamates.
A cDNA for human methenyltetrahydrofolate synthetase (MTHFS) has been isolated from a human liver cDNA library and sequenced. The nucleotide and derived amino acid sequences are unique and share a 28% amino acid homology with an E. coli protein of unknown function. The identity of the cDNA was confirmed by immunizing rabbits with a 12 amino acid peptide chosen from the derived amino acid sequence and obtaining antibodies immunoblotting against human MTHFS.
There is a test, called "CEA test", for the determination of carcinoembryonic antigen (CEA) in the serum of a patient. CEA is present in the serum of 70% of patients with colo-rectal adenocarcinoma. However, the "CEA test" does not allow for the prognosis of a treatment of a patient with an anti-tumor agent.
It would be highly desirable to be provided with a test which would allow to predict the efficacy of Leucovorin-containing chemotherapy regimens in patients with colo-rectal malignancies.